1. Field of the Invention
The present invention relates to a downhole tool for isolating zones in a wellbore. More particularly, the present invention relates to a millable bridge plug system.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
A bridge plug is a downhole tool that is lowered into a wellbore. At a particular distance through the wellbore, the bridge plug is activated. The bridge plug opens and locks to seal the bridge plug to the walls of the wellbore. The bridge plug separates the wellbore into two sides. The upper portion can be cemented and tested, separate from the sealed lower portion of the wellbore. Sometimes the bridge plugs are permanent, and they seal an entire portion of the wellbore. Other times, the bridge plugs must be removed, and still other times, the bridge plugs must be removed and retrieved. These removable bridge plugs are millable or drillable, so that a drill string can grind through the bridge plug, making remnants of the destroyed bridge plug to remain at the bottom of a wellbore or to be retrieved to the surface by drilling mud flow.
Bridge plugs generally include a mandrel, a sealing member placed around the mandrel, ring members adjacent the end of the sealing member and around the mandrel, upper and lower slip devices at opposite ends of the mandrel, and respective upper and lower cone assemblies engaged to the upper and lower slip devices. FIG. 1A shows the prior art bridge plug system 10 with a mandrel 12, sealing member 14, and upper and lower slip devices 16 and 18 shown. The bridge plug is placed in the wellbore by a setting tool on a positioning assembly, such as wireline, coiled tubing or even the drill string itself. Once in position at the correct depth and orientation, the bridge plug is activated. The setting tool holds the mandrel 12 in place, while a ramming portion of the setting tool exerts pressure on the stack, which includes the sealing member 14 and the slip devices 16 and 18. The end 22 has a cap which prevents the stack from sliding off the mandrel 12, when the ramming portion of the setting tool hits the stack. Instead, the pressure of the ramming portion compresses the stack, forcing the sealing member 14 to radially extend outward to seal against the wellbore or case and to flatten to a smaller height along the mandrel. The slip devices 16 are toothed and are distended radially outward by the stack to dig into the wellbore walls, locking the sealed configuration of the stack. The lower slip device 18 holds position by the cap at the end 22, while the upper slip device 16 lowers and locks the seal of the spread sealing member 14. When the ramming portion has compressed and locked the stack, the end 20 proximal to the setting tool on the positioning assembly is sheared, separating the bridge plug from the setting tool and the positioning assembly. FIG. 1B shows the prior art bridge plug system 10 in an activated and set state. Pressure on the lower cone assembly against the lower slip device 18 at the distal end of the mandrel causes the lower slip device 16 to open and latch against the wellbore. Continuing pressure by the ram expands the sealing member 14 against the rings to form a seal against the walls of the wellbore. Pressure on the upper cone assembly causes the upper slip device 18 to also open and latch against the wellbore, setting the seal of the sealing member.
The activation of the bridge plug requires advancement for a more efficient and stable seal in the wellbore. The ramming portion provides the force needed to form the seal on the wellbore, and this force is directed by the stack structures, the sealing member, ring members, cone assemblies, and slip devices, of the bridge plug. The interactions between these stack structures are important for efficiency and consistency of the forming the seal and locking the seal on the wellbore. The pressure is exerted directly on the sealing member by ring members in some arrangements of the stack structures. The interface between the sealing member and the ring members of the prior art has a constant taper angle between the sealing member and the ring members. The amount of pressure against the sealing member does not vary as the pressure of the positioning assembly is exerted through the ring members. The expansion of the sealing member to the wall of the wellbore is steady, yet possibly insufficient for an adequate seal. The lack of a threshold amount of pressure for setting the seal may result in a sealing member that is not expanded enough to form a good seal or extrusion of the sealing member beyond the ring members due to too much pressure. The exerted pressure on the sealing member may also be too much, causing extrusion and degradation of the seal member. There is a need for resistance to excess pressure after the seal is formed.
There is also a need for more controlled activation of the slip devices against the wellbore. The slip devices dig into the walls of the wellbore to prevent the bridge plug from slipping and to set the seal of the sealing member. The slip devices resist pressure upward and downward through the wellbore. The interface between the cone assemblies and the slip devices can result in uneven activation of the slip devices around the mandrel. One arc of the slip devices may be triggered before the entire slip device so that stresses on the cone assemblies and the slip devices are irregular and risk failure on the over-stressed portions.
Conventional materials of the millable bridge plug, like all downhole tools, must withstand the range of wellbore conditions, including high temperatures and/or high pressures. High temperatures are generally defined as downhole temperatures generally in the range of 200-450 degrees F.; and high pressures are generally defined as downhole pressures in the range of 7,500-15,000 psi. Other conditions include pH environments, generally ranging from less than 6.0 or more than 8.0. Conventional sealing elements have evolved to withstand these wellbore conditions so as to maintain effective seals and resist degradation.
Metallic components have the durability to withstand the wellbore conditions, including high temperatures and high pressures. However, these metallic components are difficult to remove. De-activating and retrieving the bridge plug to the surface is costly and complicated. Milling metallic components takes time, and there is a substantial risk of requiring multiple drilling elements due to the metallic components wearing or damaging a drilling element of a removal assembly.
Non-metallic components are substituted for metallic components as often as possible to avoid having so much metal to be milled for removal of the bridge plug. However, these non-metallic components still must effectively seal an annulus at high temperatures and high pressures. Composite materials are known to be used to make non-metallic components of the bridge plug. These composite materials combine constituent materials to form a composite material with physical properties of each composite material. For example, a polymer or epoxy can be reinforced by a continuous fiber such as glass, carbon, or aramid. The polymer is easily millable and withstands the wellbore conditions, while the fibers also withstand the wellbore conditions and resist degradation. Resin-coated glass is another known composite material with downhole tool applications. Composite materials have different constituent materials and different ways of combining constituent materials.
It is an object of the present invention to provide an embodiment of the millable bridge plug system.
It is another object of the present invention to provide an embodiment of the millable bridge plug system with improved stack structures, including cone assemblies.
It is another object of the present invention to provide an embodiment of the millable bridge plug system with improved cone assemblies.
It is still another object of the present invention to provide an embodiment of the millable bridge plug system with cone assemblies with an active interface with respective slip devices.
These and other objectives and advantages of the present invention will become apparent from a reading of the attached specifications and appended claims.